Craft Yarn Council of America April, 2003
(pointed out by LII)
The publishers, fiber, needle and hook manufacturers and yarn members of the Craft Yarn Council of America have worked together to set up a series of guidelines to bring uniformity to yarn, needle and hook labeling and to patterns, whether they appear in books, magazines, leaflets or on yarn labels. Our goal is to make it easier for consumers to select the right materials for a project and complete it successfully.
Here's a cool site to look up yarn weights and metric needles.